Conventional modems, such as V.34 modems, treat the public switched telephone network (PSTN) as a pure analog channel even though the signals are digitized throughout most of the network. In contrast, pulse code modulation (PCM) modems take advantage of the fact that most of the network is digital and that typically central site modems, such as those of internet service providers and on-line services, are connected to the PSTN via digital connections (e.g., T1 in the United States and E1 in Europe). First generation PCM modems transmit data in PCM mode downstream only (i.e., from a central site digital modem to an analog end user modem) and transmit in analog mode, e.g. V.34 mode, upstream (i.e., from the end user modem to the central site modem). Future generation PCM modems will also transmit data upstream in PCM mode.
With PCM downstream, the central site PCM modem transmits over a digital network eight bit digital words (octets) corresponding to different central office codec output levels. At the end user's central office, the octets are converted to analog levels which are transmitted over an analog loop. The end user's PCM modem then converts the analog levels, viewed as a pulse code amplitude modulated (PAM) signal, into equalized digital levels. The equalized digital levels are ideally mapped back into the originally transmitted octets and the data the octets represent.
With PCM upstream, the end user PCM modem transmits analog levels over the analog loop corresponding to data to be transmitted. The analog levels are modified by the channel characteristics of the analog loop and the modified levels are quantized to form octets by a codec in the end user's central office. The codec transmits the octets to the PCM central site modem over the digital network. The PCM central site modem determines from the octets the transmitted levels and from the levels the data transmitted by the end user PCM modem is recovered.
A difficulty that exists with upstream PCM transmission is that the levels transmitted by the end user PCM modem are modified by the analog loop. Since these modified levels are the levels that are quantized to form octets by the codec, and not the levels that are actually transmitted, it can be difficult for the central site modem to accurately determine from the octets the data being transmitted by the end user PCM modem. This difficulty is compounded by the fact that there is a channel null in the analog loop, quantization noise introduced by the codec in the end user's central office and downstream PCM echo, which make it more difficult for the central site PCM modem to accurately recover the data transmitted.
Therefore, a need exists for a device and method for precoding data signals for PCM transmission such that the analog levels that are transmitted by the end user PCM modem accurately produce predetermined analog levels (constellation points) at the input to the codec in the end user's central office, which analog levels (constellation points) correspond to the data to be transmitted by the end user PCM modem. Moreover, there is a need for a device, system and method for preceding data signals for PCM transmission which limits the transmit power and combats a channel null introduced by the analog loop and quantization noise introduced by the codec in the end user's central office.